The temperature dependence of the emission intensity of certain phosphors is known in the art. U.S. Pat. No. 8,105,502, for instance, mentions CaAlSiN3:Eu as a red luminescent material for an LED lamp (light-emitting diode). Such luminescent material, when excited in a broad (ultraviolet to blue) wavelength range, shows luminescence in the range of an orange color region to an infrared region. Among many luminescent materials, this CaAlSiN3:Eu has—according to U.S. Pat. No. 8,105,502—relatively excellent temperature characteristics. It is further indicated that it is estimated that as the power of the LED lamp is increased, the temperature of the device is further increased, and that accordingly luminescent materials are required that have more excellent temperature characteristics.
High brightness light sources are interesting for various applications including automotive lighting, such as headlamps. A method of obtaining high intensity light is based on using lasers where the high intensity laser beam is focused on a rotating phosphor wheel. It is also possible to provide a phosphor (i.e. luminescent material) on a static heat sink and illuminating it with a focused laser beam in order to obtain high intensity white light. In such an arrangement e.g. white light is obtained when blue light is partially converted to yellow light by a phosphor ceramic. The thickness and the activator concentration can be adjusted so that the ratio of converted and unconverted light is controlled. In this way the desired color point can be obtained. However, during the use of the light source, for example during dimming etc., the temperature of the phosphor can change. As the phosphor, especially a so called garnet phosphor, shows temperature dependent absorbance, the color point of the device can show a change as a result of this temperature change, which is an undesirable effect.
Hence, prior art lighting devices suffer from temperature dependency of the emission (luminescence) of the luminescent material, which especially becomes relevant when subjecting such luminescent material to high power excitations.